1. Field of the Invention
The present invention is directed to a method for producing an image employing a magnetic resonance imaging apparatus, and in particular to a method based on the turbo-spin echo sequence.
2. Description of the Prior Art
European Application No. 0175184 discloses pulse sequence wherein a 90.degree. excitation radio-frequency pulse is followed by a plurality of 180.degree. radio-frequency pulses which function as refocusing pulses. A spin echo signal follows each refocusing pulse. An image of the examination subject can be reconstructed from the acquired signal after phase-coding and read-out using known image reconstruction methods.
A sufficient number of echo signals can be acquired after a single excitation, using this known method, so that the image acquisition of a complete slice of the examination subject is possible. A pulse sequence of this type is known in the art as a "single-shot sequence." A problem with such known sequences, however, is that the amplitude of the echo signals decreases after the excitation dependent on the transverse relaxation time T.sub.2. When the echo signals are phase-coded in a conventional manner so that they are entered into the raw data matrix from the first to the last matrix line according to their sequence after the excitation, i.e., the effective phase-coding gradient proceeds from the highest negative value to the highest positive value or vice-versa, an extremely pronounced T.sub.2 weighting of the image arises. Such a pronounced T.sub.2 weighting is undesirable in many instances. It is essentially the middle line of the raw data matrix, i.e. the zero line, which is the controlling or predominant factor for the weighting of the resulting image. Because these samples which are entered in this middle line are measured relatively long after the excitation, given the aforementioned conventional sorting of the signals for entry into the raw data matrix, the pronounced T.sub.2 weighing arises. The sequence of the filling of the signals in the raw data matrix also influences the contrast of the acquired image and the presence of artifacts in the acquired image. This is described in the article "RARE Imaging: A Fast Imaging Method for Clinical MR," in Magnetic Resonance in Medicine 3, pages 823-833 (1986), as well as in the article "Phase-Encode Order and its Effect on Contrast and Artifact in Single-Shot RARE Sequences," in Medical Physics Vol. 18, No. 5, September/October 1991, pages 1032-1037. This second article also discusses the influence on the T.sub.2 weighting. It is proposed, for example, that the T.sub.2 weighting be diminished by coding the first echo signal after the excitation with a medium, negative phase-coding value, which is sequentially raised from echo-to-echo until the highest positive phase-coding value is reached. The remaining echoes are thereafter coded using the previously skipped, negative phase-coding values, beginning with the highest negative phase-coding value. This can be considered to be a chronological rotation of the echoes allocated to the zero line.
This type of filling, however, results in a considerable amplitude discontinuity in the raw data matrix between two neighboring lines. A so called "banding" artifact is thus caused. Moreover, additional artifacts occur in this method, as described in the aforementioned article.
The article "Fast Spin Echo with Use of Half Scanning," in the SMRM-Abstract Volume, 11th SMRM, Berlin, 1992, page 4524 states that more measuring time can be saved in turbo-spin echo sequences if only half of the Fourier space is scanned. The echo signals are thereby entered into the raw data matrix in the conventional way, i.e. beginning with the first signal after the excitation being entered in the first line of the raw data matrix. This article, however, does not refer to a "single-shot" sequence, but rather to a multiple excitation sequence.